(a) Field of the Invention:
The present invention relates to an illumination optical system for endoscopes.
(b) Description of the Prior Art:
In this kind of conventional illumination optical system for endoscopes, generally a cover lens is arranged in front of the exit end face of a light guide, that is, on the object side so as to diffuse the light emitted out of the light guide over a wide range. However, there have been problems that the light emitted at a wide angle out of the exit end face of the light guide will be turbulently reflected to vanish on the inner surface of the outer peripheral part of the cover lens or will be totally reflected on the inner surface of said outer peripheral part so as to be directed to the visual field center part or light guide and therefore the delivery of the light to the peripheral part of the visual field will be insufficient to obtain a uniform illumination over a wide range. Therefore, in order to solve such problems, there has been invented such endoscope optical system as is mentioned, for example, in Japanese U.M. Preliminary Publication No. Sho No. 57-170701 wherein the surface on the light guide side of a cover lens (concave lens) is made a compound surface consisting of a spherical center part and a conical peripheral part. However, therein, the direction of the principal ray of the light emitted at a wide angle out of the exit end face of the light guide is only changed but the total reflection on the inner surface of the outer peripheral part of the cover lens of the lower side ray is not yet solved. Therefore, it has been insufficient to obtain a uniform illumination over a wide range.